I. Field of the Invention
The present invention generally relates to the fabrication of integrated passive devices, and more particularly, to topologies for passive filters fabricated utilizing organic laminates.
II. Description of Related Art
Radio frequency (RF) filters are generally used to remove the out-of-band energy and perform rejection of image-band signals. The design of RF filters in most architectures is becoming a problem since center frequencies are scaling towards the multi-gigahertz range for most RF standards. As the carrier frequency becomes higher, the loaded Q (carrier frequency ÷3 dB bandwidth) for filters becomes higher, which places higher demand on the unloaded quality factor for components such as inductors, capacitors and resonators that make up the filter device.
Coaxial cavity or monoblock type filters have become very popular in commercial applications, especially in portable communication equipments, due to their high performance. Low loss is achieved in such devices with transmission line sections that are rounded, such as coax lines, or by avoiding sharp corners. However, there are several disadvantages to ceramic coaxial cavity or monoblock filters. For example, mold for these filters is expensive and each new design usually needs a new mold. Also, when fabricating coaxial type ceramic filters, different coaxial resonators are sintered and coated separately, and then connected to each other by soldering the connecting wires by hand. Yet further, such filters are typically fastened to some mounting support in a mechanically reliable manner, which adds to the difficulty and cost of the manufacturing process. Lastly, size reduction is achieved by using special high dielectric constant ceramics, resulting in a reduction of the effective wavelength in the medium.
Multilayer planar filters fabricated using multilayer ceramic (MLC) technology based on low temperature co-fired ceramic modules (LTCC), and multilayer LTCC based filters can have a volume 1/40th that of ceramic cavity filters. Such devices are being developed for data communication equipments, and digital cordless telephones, where unlike cellular applications, narrow bandwidths and large roll-offs are not required. These filters may use non-traditional metallization techniques used in ceramic technology to achieve metal thicknesses of approximately 100 μm to lower higher frequency losses. Due to other fundamental limits on the technology, the MLC and LTCC filters do not perform as well as the cavity filters. For example, one limitation is in the lack of flexibility of choosing a thickness (e.g., 4 mil<thickness<8 mil) of the dielectric sheets that make up the ceramic components. Additionally, the multilayer ceramic filters come with the disadvantage of higher costs due to the non-traditional processes used in making the multilayer ceramic filters. An example of higher costs is in the inherent higher temperatures of processing (e.g., >800° C.) compared to organic laminate processing (e.g., <230° C.). Additionally, one can leverage the economies of scale when using organic laminate processing which can handle batch processing of 18″×12″ panels as compared to a nominal maximum of 8″×8″ for LTCC and 6″×6″ for MLC technologies.
While realizing the problems with ceramic and advantages with organic laminate processing in terms of costs, filters fabricated in organic substrates generally have not delivered the performance of cavity filters or multilayer ceramic filters. The bandwidths realized by the organic filters have not been small enough and the insertion loss too high for even large bandwidth applications. See, for example, Son, M. H., Kim, Y. J., Lee, S. S, “Low-Cost Realization of ISM Band Pass Filters Using Integrated Combline Structures,” 2000 Asia-Pacific Microwave Conference, pp. 1294–1297; G. Hong and M. Lancaster, Microstrip Filters for RF/Microwave Applications Design, Wiley, June 2001.
Thus, there is an unsatisfied need in the industry for a high frequency, low loss, inexpensive bandpass filter having a relatively small footprint.